The design of marine fuel systems is regulated by several sections of the Code of Federal regulations including 33 CFR §§183.501-183.590. Fuel tank systems are required to not leak when pressure tested at 3 psig or higher if the fuel tank is rated at a higher pressure. At the same time, the fuel tank system is required to have a vent system that prevents pressure in the tank from exceeding 80 percent of the pressure marked on the tank label. 33 CFR §§183.520. This regulation defines the requirement that the vent system vent excess pressure if volatile fuels vaporize and cause a pressure buildup in the system, to reduce the chance of an explosion.
Fuel tanks in boats commonly have a closable fuel cap, an inlet, a connecting pipe or hose, and a remote fuel tank which receives fuel via the pipe or hose. Fuel is pumped into the inlet from a gasoline or diesel fuel pump through the pump nozzle and then flows by gravity via the pipe or hose to the fuel tank.
A common problem in filling such marine fuel storage systems with fuel is that fuel entering the tank displaces air in the empty tank, causing an outflow of air from the tank, up the hose or pipe and out of the inlet. The flow of displaced air bubbles up through the incoming fuel, causing blockages and instabilities in the fuel flow. The periodic bubbles disrupt fuel flow in the pipe or hose, and can cause fuel to be pushed back. These problems are most common in motorboat and sailboat fuel storage systems where it is common for the gas inlet to be placed on a horizontal surface, with the pipe or hose to be positioned vertically. Entering fuel travels downwardly, and exiting air travels upwardly. With both streams in the same passage each stream impedes each other's flow. A bubble of air moving upwardly can push on the incoming fuel and causes fuel to splash or spill out of the inlet. This problem is commonly referred to as “spitback.” Note that the fuel tank vent system required by 33 CFR §§183.520 is not effective to release air in the tank during filling because the pressure is usually insufficient to open the vent system.
Furthermore, it noted that prior to 2009, marine fueling stations were not required to have the full tank fuel flow cutoff which is standard in automobile fueling systems. While full tank fuel flow cutoffs are now standard, they are not effective to cutoff fuel flow when they encounter a bubble of air and fuel typical of a spitback condition.
It is to be appreciated that the extent of the spitback problem varies widely in both existing vessels and new vessels. Depending on the particular design of the fuel system layout there may be no problems at all if there is a larger diameter angled fuel pipe leading from the inlet to the fuel tank which allows the streams of air and fuel to separate and travel in separate paths; but if there is a smaller diameter fuel pipe or a vertical fuel pipe or a fuel pipe with many bends then the problem of spitback is an inevitable problem.
40 CFR 1060.101(f)(3) became effective on or before Jul. 31, 2011 and attempts to provide a design requirement for all fuel systems that the systems be designed to avoid spitback:                Refueling. For any equipment using fuel tanks that are subject to diurnal or permeation emission standards under this part, you must design and build your equipment such that operators can reasonably be expected to fill the fuel tank without spitback or spillage during the refueling event. The following examples illustrate designs that meet this requirement:        (i) Equipment that is commonly refueled using a portable gasoline container should have a fuel tank inlet that is larger than a typical dispensing spout. The fuel tank inlet should be located so the operator can place the nozzle directly in the fuel tank inlet and see the fuel level in the tank while pouring the fuel from an appropriately sized refueling container (either through the tank wall or the fuel tank inlet). We will deem you to comply with the requirements of this paragraph (f)(3)(i) if you design your equipment to meet applicable industry standards related to fuel tank inlets.        (ii) Marine SI vessels with a filler neck extending to the side of the boat should be designed for automatic fuel shutoff. Alternatively, the filler neck should be designed such that the orientation of the filler neck allows dispensed fuel that collects in the filler neck to flow back into the fuel tank. A filler neck that ends with a horizontal or nearly horizontal segment at the opening where fuel is dispensed would not be an acceptable design.        
However, this new regulatory requirement does not alter the spitback problem experienced with many preexisting boats and other marine vessels. And even new vessels can conceivably still experience the problem.
The spitback splashes and spills of fuel can be an environmental problem. A fuel spill into a waterway is a violation of Clean Water Act regulations. Furthermore, a spill in a marine environment is difficult to contain; it is often difficult to capture the spilled fuel and to prevent the spread of spilled fuel. A fuel spill can cause other economic damage, for example, damage the marine vehicle, docks, or filling station. There are other potential risks such as fire hazards and damage to human health from exposure to gasoline.
There is a need for a nonspill fuel filler adapter and a nonspill fuel filler inlet to enable transfer of fuel from a fuel pump via a fuel nozzle to a fuel fill/pipe or hose/tank that reduces or prevents spillage. There is a need for a device that minimizes the volatile and harmful emissions of fuel vapor during the fueling process to reduce the risk of fire and exposure to hazardous substances. There is a further need for a device that cooperates with the automatic shut-off systems of fuel dispensers to maintain their ability to prevent overflow.